The concept of monitoring liquid such as water in a container to determine its depth or level in terms of some predetermined level and providing an electrical signal to indicate said change of level is not new. Many methods have been taught such as floats, thermocouples, sonic and temperature sensors, light reflections means, and other complicated mechanical apparatus and electrical circuits and components involving radioactive means, heat streams, amplifiers, etc. In the prior art a great amount of mechanical apparatus and electrical components and circuits have been used which require a large amount of space and expensive monitoring equipment which requires maintenance and qualified personnel to operate. Such level sensing arrangements and apparatus are also subject to signal error because of varying heat flow and temperature conditions of the liquid between measurement zones. Such signal errors are not capable of being reliably corrected by accumulated error data because of the often unpredictable nature of heat flow conditions within the liquid body being monitored.